Electroluminescent polymers are materials that emit light when sandwiched between two suitable electrodes and when a sufficient voltage is applied. A number of electroluminescent devices have been disclosed which use organic materials as an active light-emitting layer sandwiched between two electrodes. For example, VanSlyke et al. in U.S. Pat. No. 4,539,507 disclose a device having a bilayer of two vacuum-sublimed films of small organic molecules sandwiched between two contacts. The small organic molecules however are not printable using a solution-based process. In a related patent, Friend et al. in U.S. Pat. No. 5,247,190, disclosed a device having a thin dense polymer film made up of at least one conjugated polymer sandwiched between two electrodes. Subsequently, Braun at al. in U.S. Pat. No. 5,408,109 showed that high brightness light emitting devices could be made using soluble electroluminescent polymers. Their results indicated that it may be possible to make light emitting displays using inexpensive solution-based atmospheric processing techniques, such as ink jet printing, reel-to-reel or screen printing; however, efficient device operation required the use of low work-function metals, such as Ca, that are not stable under atmospheric processing (i.e. printing) conditions.
Pei et al. describe a polymer light-emitting electrochemical cell (U.S. Pat. No. 5,682,043) which contains a solid state electrolyte and salt that is used to electrochemically dope an organic electroluminescent layer, such as a conjugated polymer, via ionic transport. This system provides the ability to achieve efficient device operation without relying on the use of low work-function metals. Following this work, Cao showed in U.S. Pat. Nos. 5,965,281 and 6,284,435 that organic anionic surfactants cause a similar effect without needing ionic transport through the polymer film. The patents described in this paragraph disclose many anions and cations that are useful in the present invention, and their disclosures are incorporated herein by reference. In theory, electrochemical doping or anionic surfactants could be used to make an electroluminescent polymer device that would be fully compatible with liquid-based processing under atmospheric conditions. Nonetheless, the electroluminescent polymer solutions mentioned in these patents are not easy applicable to many fully liquid-based manufacturing process, such as screen printing and gravure, and also have limited lifetimes.
Screen printing is one of the most promising methods to inexpensively manufacture large-area electroluminescent displays. Screen printing has been successfully applied to manufacturing large area inorganic phosphor-based electroluminescent displays by Topp et al. in U.S. Pat. No. 4,665,342. Victor et al. later showed that screen printing, and related printing techniques, can be used to manufacture polymer-based electroluminescent displays (U.S. Pat. No. 7,115,216) using a fully printable cathode. Carter et al. (U.S. Pat. No. 6,605,483) revealed a method to make a printable electroluminescent ink that improves the screen printability and performance of electroluminescent polymer solutions through the use of soluble or dispersible additives, such as gel retarders, high boiling point solvents, and ionic dopants. More recently, flexographic, gravure, capillary, nozzle, slot and spray deposition techniques have been employed to deposit organic electronic inks in similar structures including electroluminescent, photovoltaic and sensing devices. Nonetheless, these inks still suffer form lower lifetimes when fully printed.